A wide-scale study published on May 14 has strengthened warnings, spelt out last year by UN scientists, that climate change is already on the march. The paper, published in Nature, goes beyond the scope taken by a landmark report issued by the UN's Intergovernmental Panel on Climate Change (IPCC) in February 2007.
In that document, the IPCC said man-made global warming was "likely" - within a probability of 66-90 percent - to have had a "discernible" effect on many physical and biological systems. The new study, published in the British journal Nature, is written by many of the people who wrote the so-called Working Group I report, the first of a trio of major assessments released last year by the IPCC.
Its approach widens the net of data for making a fresh analysis. It concludes "significant changes" are already occurring among natural systems on all continents, with the exception of Antarctica, and in most oceans.
"Humans are influencing climate through increasing greenhouse gas emissions," said lead author Cynthia Rosenzweig of the NASA Goddard Institute for Space Studies and the Columbia Center for Climate Systems Research.
"The warming world is causing impacts on physical and biological systems attributable at the global scale." The analysis is based on a trawl of hundreds of papers published in peer-reviewed journals, on data stretching back to 1970s.
These investigations covered phenomena as varied as the earlier leafing of trees and plants; the movement of species to higher latitudes and altitudes in the northern hemisphere in response to warmer weather; the shrinkage of glaciers and melting of permafrost; and changes of bird migrations in Europe, North America and Australia.
Critics of the IPCC report have variously argued that the perceptible warming that has occurred over the last three decades is due to natural causes, such as volcanic eruptions or changes in solar radiation, or to the effect of deforestation and other changes in land use. The new paper rejects this, saying the changes in Earth's natural systems cannot be explained by such factors, and only man-made warming could be the culprit.
The Working Group I report forecast likely warming of 1.8-4.0 degrees Celsius (3.2-7.2 degrees Fahrenheit) by 2100 and a rise in sea levels of at least 18 centimetres (7.2 inches). Hunger, homelessness and water-borne disease are among the many risks that would be amplified as a result of climate change.
In a commentary, also published by Nature, climatologists Francis Zwiers and Gabriele Hegerl picked over the big dispute as to whether climate impacts could be pinned on human interference.
They placed a question mark over the shortness of the records put forward by Rosenzweig's team. Evidence stretching back far longer than a few decades was needed to get a solid perspective, they said. But, they added, these objections are outweighed by "the sheer number of changes" that the paper lists.
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